1. Field of the Invention
The present invention relates to the field of electro-proportional valves.
2. Prior Art
Electro-proportional valves are well known in the prior art. These valves provide electrically variable pressure regulation responsive to an input current. By way of example, Hydraforce, Inc. of 500 Barclay Blvd, Lincolnshire, Ill. manufactures Proportional Pressure Control, Reducing/Relieving, direct-Operated valves of this type, such as their EHPR98-T33 valves. These valves are direct-operated or single stage valves. Two stage valves of this general type are also commercially available, such as the Proportional Pressure Reducing Valves available from Eaton Vickers of 14615 Lone Oak Road, Eden Prairie, Minn. Whether single stage or two stage, such valves generally balance the magnet force generated by a solenoid within the valve with a pressure feedback to control the pressure. The HydraForce valves referred to are drop-in valves, meaning that the valves include a valve spool and are adapted to drop into a valve body provided by their customer, whereas the Vickers valves referred to are complete valve units.
The primary disadvantage of the prior art valves of this general type is that the solenoid utilized to control the force balance on the flow control spool requires a continuous electrical current in order to generate force and thus regulate pressure. This is inefficient and generates unnecessary heat. Heating of the solenoid changes the electrical resistance and creates hysteresis in the commanded pressure versus current relationship for the valve. Heat introduced into the working fluid by the valve must be removed from the fluid elsewhere in the system, increasing the system's cooling requirements.